Trump has said he is currently under audit and won't release the documents until the audit wraps.

But being under audit does not prevent anyone from releasing their returns. And Trump's refusal to make the documents public has led to questions from both sides of the aisle about what he may be hiding.

That includes Clinton's campaign, which released a web video Friday speculating what lies in the withheld tax returns.

TheNew York Times reported Thursday that Trump may not have paid any tax at all, despite claiming to be a billionaire. The Times report said Trump may have taken advantage of tax credits for developers that would bring his total taxable income to $0 — meaning he would pay less than middle-class and low-income families.

A tax return would also show how much money Trump makes a year. Trump has claimed to be worth $10 billion, but his annual income could be far less.

The returns would also show how much money Trump has donated to charity. Trump claims to donate to charitable organizations, but Washington Post reporter David Farenthold has found little evidence to support Trump's charitable donation claims.

A reminder: Trump continues to hide his tax returns from us.pic.twitter.com/mRwho7GnaE

Finally, a tax return could show what kind of financial ties Trump has to foreign countries — including Russia, which has seemingly sought to meddle in the 2016 election through a possible email hack of numerous Democratic campaign entities.

Those ties could be revealed, as Americans can claim tax credits for any money earned abroad.